Apparatus for locating concealed or buried metal bodies and a stable inductor usable in such detectors

ABSTRACT

A metal detector for locating metal bodies buried in the ground or embedded in other structures having a probe assembly which is movable parallel to the surface of the earth or such structure and whose inductance varies as the inductor assembly is moved into the proximity of such concealed object, the detector providing an output signal whose frequency varies in accordance with the inductance of the probe assembly, wherein the probe assembly includes large diameter and small diameter inductor assemblies selectively connectible in a frequency determining circuit of an oscillator and wherein the signal produced by the detector may be indicated visually by means of a meter, audibly by a speaker to which the output of the detector is transmitted, or by neon bulbs mounted on the inductor assemblies which light up with a frequency which varies in accordance with the frequency of the output of the detector. A stable inductor assembly usable in detectors of the type described and having an inductor coil formed of a coaxial cable whose inner conductor is connected in the frequency determining circuit of an oscillator and whose outer coaxial conductor is grounded to form a partial Faraday shield between the inner conductor and a Faraday shield mounted exteriorly of the coil.

United States Patent Garrett 1 May 9,1972

[54] APPARATUS FOR LOCATING CONCEALED OR BURIED METAL BODIES AND ASTABLE INDUCTOR USABLE IN SUCH DETECTORS Charles L. Garrett, 1201 RavinaDrive, Garland. Tex. 75040 221 Filed: Apr. 13,1970

211 Appl. No.: 27,575

[72] Inventor:

Primary E.\'aminerGerard R. Strecker Attorney-Walter J. J agmin [5 7]ABSTRACT A metal detector for locating metal bodies buried in the groundor embedded in other structures having a probe assembly which is movableparallel to the surface of the earth or such structure and whoseinductance varies as the inductor assembly is moved into the proximityof such concealed object, the detector providing an output signal whosefrequency va ries in accordance with the inductance of the probeassembly. wherein the probe assembly includes large diameter and smalldiameter inductor assemblies selectively connectible in a frequencydetermining circuit of an oscillator and wherein the signal produced bythe detector may be indicated visually by means of a meter, audibly by aspeaker to which the output of the detector is transmitted, or by neonbulbs mounted on the inductor assemblies which light up with a frequencywhich varies in accordance with the frequency of the output of thedetector.

A stable inductor assembly usable in detectors of the type described andhaving an inductor coil formed of a coaxial cable whose inner conductoris connected in the frequency determining circuit of an oscillator andwhose outer coaxial conductor is grounded to form a partial Faradayshield between the inner conductor and a Faraday shield mountedexteriorly of the coil.

12 Claims, 9 Drawing Figures PATENTEDHAY 9 I972 3, 662,255

SHEET 1 [IF 2 F i g. 5

INVENTOR.

Charles L.GorreH ATTORNEY PA'TENTEDMAY 9 I972 SHEET 2 BF 2 Gnd.

STABLE /2a CRYSTAL 7 OSCILLATOR 29 INVENTOR.

Charles L. Garrett ATTORNEY APPARATUS FOR LOCATIN G CONCEALED OR BURIEDMETAL BODIES AND A STABLE INDUCTOR USABLE IN SUCH DETECTORS Thisinvention relates to devices for locating metal objects buried in theground or concealed in other structures and to a stable characteristicinductor assembly.

Conventional devices for detecting or locating concealed metal objectsas, for example, buried in the ground or embedded in a wall or otherbuilding structure, have a stable reference frequency oscillator and asecond oscillator the frequency of whose output varies with theeffective inductance of an inductor assembly having a ring shaped coilformed of several turns of on insulated conductor and a Faraday shieldmounted thereon, the coil being connected in the frequency determiningcircuit, such as an LC tank circuit, of the second oscillator. Thefrequency of the output of the second oscillator is compared with thefrequency of the output of the reference oscillator to provide thedetector output signal. The Faraday type electrostatic shield isprovided to prevent stray or ground capacitances from varying theeffective inductance of the coil and thus producing various unwantedchanges in the frequency of the output of the second oscillator. It isfound that the effective inductance of such inductor assembly variesconsiderably with variations in temperature at the location of theinductor assembly, probably due to the fact that a capacitance existsbetween the Faraday shield and the inductor coil and that suchcapacitance varies probably because variations in temperature cause theFaraday shield to move closer or farther away from the coil turns, thefact that the diameter of the coil may change or the fact that thespacing between the turns of the coil may change. Thus the output of thedetector will vary as such inductor assembly is moved, for example, fromshadow into the sunlight or vice versa. This Faraday shield, however, isnecessary for the proper operation of the detector. It is desirabletherefore that a detector be provide which will operate properly eventhough its inductor assembly is subject to considerable variations intemperature during use.

It is also found that in some applications, for example, in the use ofthe detector to locate objects buried in the ground below a body ofwater, the use of an earphone speaker or a meter to indicate the outputof the speaker is impractical due to extraneous noise in the first caseand the necessity for closely observing the ground at the location ofthe inductor assembly in the second case.

It is an object of this invention to provide a new and improved detectorfor locating concealed objects having an inductor assembly which ismovable adjacent the surface of the ground or a building structure andwhich will also provide a signal when the probe assembly is moved to theproximity of an object buried in the ground below the surface orembedded in such structure.

Still another object is to provide a detector of the type describedwhose inductor assembly includes a ring shaped inductor coil having aFaraday shield disposed about its bottom portion for shielding theinductor coil from stray and ground capacitances which would vary theeffective inductance of the inductor assembly and whose effectiveinductance will not vary appreciably with changes in temperature of theinductor assembly.

Another object is to provide a detector of the type described having afirst reference oscillator which provides a stable reference frequencyoutput and a second oscillator the frequency of whose output varies inaccordance with the effective inductance of an inductor assembly whichin turn varies with the proximity of metal objects thereto.

A further object is to provide a detector of the type described whereinthe inductor assembly is connected in the frequency determining circuitof the second oscillator.

A still further object is to provide a detector of the type describedwherein the effective inductance of the inductor assembly does not varywith changes of temperature.

A further object is to provide a detector having a visual means forindicating the output.

Still another object is to provide a detector of the type describedwherein the output indicating means comprises a plurality of neon bulbswhich are mounted on a circular inductor assembly.

An important object of the invention is to provide a stablecharacteristic inductor assembly whose effective inductance does notvary with variations in temperature.

Another object is to provide an inductor assembly having a circular coilformed of a plurality of turns of a coaxial cable comprising a centralor inner conductor, a cylindrical or outer conductor coaxial with andspaced and insulated from the inner conductor and an insulating sheathexteriorly of the outer conductor; and a Faraday shield disposed aboutthe coil.

Still another object is to provide an inductor assembly of the typedescribed wherein the Faraday shield and the outer conductor do not formclosed conductive paths and the shield, the outer conductor and one endof the inner conductor are all connectible to ground to prevent stray orground capacitances from changing the effective inductance of theinductor assembly.

Additional objects and advantages will be readily apparent from thereading of the following description of a device constructed inaccordance with the invention, and reference to the accompanyingdrawings thereof, wherein:

FIG. 1 is a perspective view of a detector embodying the invention;

FIG. 2 is an exploded view of an inductor coil of an inductor assemblyof the detector;

FIG. 3 is a fragmentary view, with some parts removed, of coaxial cableof which the coil of FIG. 2 is formed;

FIG. 4 is a perspective view of a conductor assembly which includes thecoil of FIG. 2 and a Faraday shield mounted on the coil;

FIG. 5 is an enlarged view, with some portions broken away, showing themanner in which the outer conductor of the cable of which the coil isformed into two sections of equal lengths;

FIG. 6 is an enlarged sectional view taken on line 66 of FIG. 1;

FIG. 7 is a diagramatic illustration of the electrical circuit of thedetector;

FIG. 8 is a diagramatic illustration of a modified form of the electriccircuit of the detector; and,

FIG. 9 is a fragmentary perspective view of a modified form of theinductor assembly embodying the invention.

Referring now particularly to FIGS. 1 through 7 of the drawings, thedetector 10 embodying the invention includes a probe assembly 11pivotally mounted on a handle 12. The lower flat end portion 13 of thehandle extends between the upright legs of a pair of brackets 14 and 15and is pivotally secured thereto by means of a bolt 16 which extendsthrough aligned apertures in the upright legs of the angle members andthe end portion 13. The angle members are secured to a connector portion18 of the probe assembly II by means of studs 19 whose lower ends areembedded in the connector portion and which extend upwardly throughsuitable apertures in the horizontal flanges of the angle members andsecured thereto by nuts threaded thereon. A housing 22, mounted on theupper inclined portion 23 of the handle, houses a battery (not shown)for providing an electric current for the detector, a loudspeaker 24, ameter 25, a switch 26 for connecting the meter in the detector circuit,a rotatable control knob 27 for varying the amplitude of the sound beingtransmitted by the speaker, a second rotatable control knob 28 forvarying the value of a variable capacitor to be discussed below, anon-off switch 29, and a rotatable probe assembly selector switch 29a.

The top end portion of the handle 12 may be curved forwardly and beprovided with a hand grip 30.

It will be apparent that the probe assembly 11 may be pivotally adjustedto a position wherein it will extend parallel to the surface beneathwhich buried or concealed metal objects are to be sought.

The probe assembly 11 includes an outer inductor assembly 32 and aninner inductor assembly 33. The outer ends of the connector portion ofthe probe assembly are integral with the outer inductor assembly and theinner inductor assembly being integral with the connector portion 18 andthe outer inductor assembly.

As will be explained in greater detail below, the connector portion 18and an outer coating of the indicator assemblies 32 and 33 may bemoulded as a single unit and be formed of fiber glass, and the like.

The outer inductor assembly 32 includes a coil 40 formed of severalturns of a coaxial cable 43. In the present instance, the coil 40 isshown as having only two turns 41 and 42 for clarity of illustration. ltwill be apparent to those skilled in the art that such coil 40 may beformed of as many turns as desired or as is required for a particularapplication.

The coaxial cable includes a central or inner solid conductor 45, acylindrical sheath conductor 46 coaxial with the conductor 45 formed ofa woven conductive material, such as copper strands, and separated fromand held in coaxial relation relative to the central conductor 45 by acylindrical insulation 47. The cable 43 also includes an outercylindrical coating or layer of insulation 48.

For reasons to be explained below, the cable 43 prior to its windinginto the coil 40 has a small central section of the outer conductor 46removed, by any suitable means, to divide it into two separate sections51 and 52 of the outer conductor 46. The conductor 46 is easily dividedinto the two separate sections by cutting out a central section of theouter insulation coating 48 and of the outer conductor 46 to form theannualar external recess 53in the conductor.

After the coaxial cable is wound into the coil 40, the turns 41 and 42are secured to one another and held in circular form by a flexibleinsulation 49 wrapped about the turns. Such insulation may have anadhesive coating to hold it in place on the coil 40. The insulation ispreferably overlapped so that during the molding process in which theouter casing 50 of the probe assembly is formed, the plastic materialwill not penetrate inwardly of the insulation layer 49.

The probe assembly 40 includes a Faraday shield 54 formed of aluminumfoil or other suitable nonmagnetic, preferably malleable, substance andis secured to the bottom portion of the coil and the insulation 49 inany suitable manner, either by an adhesive or bonding agent or byshaping the malleable foil to the outer configuration of the coil. Asseen particularly in FIG. 4, the ends of the Faraday shield 54 areseparated, as at 55.

The opposite or remote ends of the outer conductor sections 51 and 52are connected to one another by the conductors 61 and 62 and to theground conductor 63 of the power input circuit 64 of the detectorcircuit 66 (FIG. 7) which is suitably mounted in the housing 22. Aconductor 68 which is connected to the midpoint of the Faraday shield 54is also connected to the ground conductor. While in FIG. 7 theconductors 61 and 68 are separately connected to the ground conductor 63for clarity of illustration, it will be apparent that the conductors 61,62 and 68 and a conductor 69 which connects to one end of the centralconductor 45 may be connected to a single insulated conductor 70 ofmulti-conductor cable 71. The opposite end of the central conductor 45may be connected to another conductor 72 of the cable. The cable may bewound about the column 12 and led into the housing through a suitablefitting 74.

The indicator assembly 33 being identical to the inductor assembly 32except for the dimension of its coil, its elements have been providedwith the same reference characters, to which the subscript a has beenadded, as the corresponding elements of the inductor assembly 32.

The inner conductors 45 and 45a of the conductor assemblies 32 and 33are selectively connectable in the LC tank circuit of an oscillator 80of the Colpitts type by means of the switch 290. The oscillator 80includes a transistor 81 whose emitter is connected to the positivevoltage conductor 82 which is connected to the positive side of a directcurrent input circuit 64 when the switch 29 is moved to its closedposition where it engages the stationary contact 83, by a conductor 85,a current limiting resistance 86, and a conductor 87.

The collector of the transistor 81 is connected by a conductor 88 to theswitch 29a. The switch 290 when it is in the position illustrated inFIG. 7 engaging its stationary contact 89 is connected to one side ofthe conductor 45 of the coil 40 by the conductor 90 and the collector ofthe transistor is connected to the ground conductor 63 through theconductor 45 of the coil 40. When the switch is in its opposite positionwhere it engages its other stationary contact 91, the collector of thetransistor is connected to the ground conductor 63 through through theconductor 45:: of the other small diameter inductor assembly 33.

The inductances of the two inductor assemblies 32 and 33 are equal sothe frequency of the signal output of the oscillator 80 will be the samewhen the switch is in either of its two operative positions, i.e., ifeither the inductor assembly 32 or the inductor assembly 33 is connectedin the LC tank circuit of the oscillator 80.

The frequency of the output of the oscillator 80 varies in accordancewith the inductances of the coils formed by the conductor 45 or 45a ofthe coils 40 and 40a, respectively, while its normal value is determinedby values of the capacitors 94, 95, and 96, as well as the value of theinductance of such coil 40 or 40a connected to form with such coil theLC tank circuit of the oscillator 80.

The capacitors 94 and 95 are connected in series between the groundconductor 63 and the collector of the transistor 81 by means of theconductors 98, 99, 100, and 87 while the variable capacitor 96, whosecapacitance may be adjusted by the central knob 28, is connected betweenthe ground conductor and the emitter of the transistor 81 by theconductors 103, 104, and 87.

The common connection 106 of the capacitors 94 and 95 is connected tothe common connection 107 of the resistance 86 and the emitter of thetransistor 81 by means of the conductor 109 which is connected to theconductors 85 and 99. The base of the transistor 81 is connected by aconductor 110 to the common connection 111 of a pair of resistances 112and 113 which form a voltage divider and which are connected in seriesbetween the ground conductor 63 and the positive voltage conductor 82 bythe conductors 115, 116, and 1 17.

The output of the oscillator 80 is transmitted to the input terminal 119of a mixer circuit 120, the input terminal 119 of the mixer circuitbeing connected to the common connection of the resistance 86 of thecollector of the transistor 81 by a conductor 121.

it will be apparent that for any given value of the variable capacitor96, since the capacitors 94 and 95 have fixed values, the frequency ofthe output of the oscillator 80 will vary as the inductance of the coilof the inductor assembly 32 or 33 which is at that time connected by theswitch 29a in the LC circuit of the oscillator 80.

The detector circuit 66 also includes any suitable stable referencefrequency oscillator 125, for example, an oscillator the frequency ofwhose output is stabilized by a crystal, as is well known to thoseskilled in the art.

The reference frequency oscillator is connected across the ground andpositive conductors 63 and 82, respectively, by the conductors 126 and127 while its output is transmitted by a conductor 128 to the otherinput terminal 129 of the mixer circuit 120. The mixer circuit is of anysuitable type which, when the frequency of the output of the oscillator80 varies from the frequency of the oscillator 125, provides adifference or beat frequency which is equal to the difference in thefrequencies of the signals applied to its input terminals 129 and 119.For example, if the frequency of the output of the oscillator 80 is 30cycles per second greater than the frequency of the output of thereference frequency oscillator, the mixer transmits a 30-cycle persecond signal to the input terminal 132 of an audio amplifier and signalconditioning circuit 133. The gain of the circuit 133 may be varied byvarying a suitable variable resistance of the audio amplifier circuit byrotation of the knob 27.

The power voltage, of course, is applied to the circuit 133 from thevoltage input circuit 64 when the switch 29 is in its closed position,the audio amplifier being connected to the ground conductor 63 by theconductor 135 and to the positive voltage conductor 82 by the conductor136. Biasing voltage is supplied to the audio amplifier by means of aconductor 138 which is connected to the common connection 111 of theresistances 112 and 133.

The audio amplifier produces an audio frequency signal which may beapplied to the speaker 24, one side which is connected by the conductor140 to the output circuit of the audio amplifier and whose other side isconnected to the positive voltage conductor 82 by the conductor 141. Theoutput of the circuit 133 may also be applied across the meter 25, whenthe switch 26 is in closed position, since the meter then is connectedto the output circuit of the circuit 133 by the conductor 142 and to thepositive voltage conductor 82 by the conductor 143.

It will be noted that the Faraday shield 54 does not form a closedconductive path due to the provision of the gap 55 therein and that itis connected to ground at the midpoint thereof remote from and directlyopposite such gap in order that no unbalance in capacity effects beintroduced into the inductor assembly. Similarly, the outer conductor 46does not provide a closed conductive path since the outer centralsection of the outer conductor is removed to form the sections 51 and 52of equal lengths and the ends of the sections 51 and 52 remote from thecentral gap are also connected to ground so that no unbalance incapacity effects is introduced thereby into the inductor assembly.

It would, of course, be possible to connect the ground conductor to thecentral section of the outer conductor instead of removing such centralsection in which case the opposite ends of the outer conductor would notbe connected to one another or to any of the outer parts of the circuit.

In use, when it is desired to locate a metal object buried at arelatively great depth in the ground, the switch 29a is moved to theposition illustrated in FIG. 7 to connect one end of the coil formed bythe inner conductor 45 of the coil 40 in the LC tank circuit of theoscillator 80 since the electromagnetic field produced by the largediameter coil of the inductor assembly 32 will penetrate further intothe ground than the electromagnetic field produced by the small diameterinductor assembly 33. The on-off switch 29 is then moved to its closedposition, the probe assembly is moved to a position closely adjacent tothe ground and parallel thereto and the variable capacitance 96 isadjusted by means of the knob 28 to cause the frequency of the output ofoscillator 80 to be exactly equal to the frequency of the output of thereference oscillator 125. As a result, the mixer will not provide anysuch output which would be detected by the ear of the operator since itwould cause an audible sound to be generated by the speaker each time apulse is produced by the mixer circuit. Alternatively, the frequency ofthe output of the oscillator 80 may be made slightly different than thereference frequency so that a constant low frequency output is produced.

The operator then slowly moves the probe assembly parallel to the groundand thus sweeps a desired area of the ground. As the probe assemblymoves into the proximity of a metal object or other conductivesubstance, such as iron or mineral, the magnetic field produced by thealternating current in the conductor 45 by the operation of theoscillator 80 is disturbed by such conductive metal object or substanceand the inductance of the coil formed by the inner conductor 45 changes.Such change in the effective inductance of the conductor 45 connected inthe LC tank circuit of the oscillator 80 causes a corresponding changein the output of the oscillator 80 which now becomes either greater orsmaller depending upon the particular characteristics of the buriedobject, such as its size. As a result, the frequency of the output ofthe oscillator 80 now becomes different from the frequency of the outputof the reference oscillator and the mixer provides an output signal tothe audio amplifier whose frequency varies in accordance with thedifference between these frequencies. This change in iductance becomesgreater as the inductor assembly 32 moves directly over such buriedobject and becomes greatest when the inductor coil is exactly centeredover the buried object and the object is located at the central axis ofthe inductor assembly 32. The operator then knows exactly where to digto uncover the object.

Should it be desired to search for small or large objects which are notburied to any great depth below the surface of the ground, for example,not more than 2% feet below the surface of the ground, the switch 29a ismoved to its other opposite position where it connects the collector ofthe transistor 81 to the end of the inner conductor 45a of the innerassembly 33. The magnetic field produced by the small diameter inductorassembly penetrates less deeply into the earth, but is of greaterintensity and therefore can detect smaller objects than the lessintense, but more deeply penetrating magnetic field produced by thelarge diameter inductor assembly 30.

During the movement of the probe over the surface of the earth, as theoperator sweeps a given area, the inductor assembly may be moved fromdirect sunlight into shadows thus causing the temperature to which theinductor assemblies are subjected to vary considerably. Such changes inthe temperature, however, will not cause any appreciable change in theeffective inductance of either of the inductor assemblies 32 or 33 forthe reasons explained above and the operator does not therefore have toadjust the capacitance 96 each time the inductor assemblies aresubjected to temperature changes as would be the case with conventionaldetectors.

Referring now particularly to FIGS. 8 and 9 ,the modified form of theinductor assembly 324 is substantially similar to the conductor assembly32 and, accordingly, its elements have been provided with the samereference numberals, to which the transcript a has been added, as thecorresponding elements of the conductor assembly 32. The indicatorassembly 320 differs from the inductor assembly 32 only in that aplurality of neon bulbs 200 are embedded in the translucent casing 50a.The neon bulbs provide a visual indication of the frequency of output ofthe mixer and will flash or glow each time a voltage pulse is appliedthereacross.

As illustrated in F IG. 2, in order that the output of the audioamplifier 133 be of such voltage and current as to cause the lamps 200to flash or glow each time a pulse is delivered by the audio amplifier,the output of the audio amplifier is applied to the base of an amplifiertransistor 201 whose emitter is connected to the positive voltageconductor 82 through the conductor 203, a resistance 204 and a conductor205. A biasing potential is also supplied to the base of the transistor,it being connected to the positive voltage conductor 82 through theconductors and 206, a resistance 207 and a conductor 208.

The collector circuit of the amplifier transistor 201 is connected inseries with the primary winding 210 of a transfonner 211, the collectorof the transistor 201 being connected to one side of the primary winding210 by the conductor 63 by a conductor 214. The bulbs 200 are connectedto one side of the secondary winding 216 of the transformer 211 by theconductors 218, a conductor 219, a resistance 220, and the conductor 221and to the other side of the secondary winding by the conductors 224 anda conductor 225.

It will now be apparent that the transistor 201 acts as an amplifier forthe output of the audio amplifier and the transformer 211 acts as avoltage step up transformer to raise the voltage of the output signal ofthe amplified output of the audio amplifier 133 to a value sufficientlyhigh to cause the lamps to flash each time the audio amplifier providesan output pulse.

The conductor assembly 32a is used in metal detectors which are usedunder water to discover objects buried or in the ground below thesurface of a body of water or lying thereon and covered by plants andthe like. in the use of such detectors under water it is vitallyimportant for the operator to continuously observe the area of thesurface of the earth near and within the circular conductor assembly32a. The operator therefore cannot use such detector as effectively ifhe must keep his eyes upon a meter rather than upon the ground beingsearch. The use of the earphone as a speaker also is not verysatisfactory because of extraneous noises in the water and the fact thatthe cable which leads from the earphone to the housing of the detectorcauses great inconvenience and may foul upon other objects or on thebody of the operator himself. The provision of the neon bulbs in theupper portion of the oscillator assembly 320 enables the operator toconstantly and continuously observe the surface near and within the areadirectly below the circle of the conductor assembly 32 a. The flashingof the bulbs which increases the frequency as the conductor assembly ismoved over the location of a metal object is readily perceived by theoperator even though he continues at all times to observe and inspectthe surface of the earth located directly below the inductor assembly32.

The indicator assembly 32a could, of course, not be mounted on a longhandle, but on a small dimension housing held directly by the operator.The neon bulbs could, of course, be mounted on such housing if desired.

It will be seen that the metal detectors embodying the invention may beused not only to discover and locate metal objects buried in the earth,but also to discover objects which may be embedded in walls of buildingstructures and the like.

it will now be seen that a new and improved metal detector for locatingconcealed metal objects has been illustrated and described which has twoinductor assemblies, one of small diameter and the other of largediameter, which provide electromagnetic fields of different intensitiesand areas so that the detector may be used effectively to locaterelatively large objects buried relatively deep in the earth and smalland large objects buried at a relatively small depth in the ground.

It will further be seen that each inductor assembly is formed of acoaxial cable whose outer conductor prevents stray variations in thecapacitance introduced by the Faraday shield of the assembly orvariations in the inductance introduced by changes in the diameter ofthe coil or the spacing of the turns of the coil caused by temperaturevariations from varying the effective inductance of the inductorassembly and thus the operation of the detector is facilitated byminimizing or eliminating the necessity for adjusting the normalfrequency of oscillation of the oscillator 80 upon occurrence oftemperature changes.

It will also be seen that the spacing between the inner and outerconductors 45 and 46, respectively, of the coil 40 is held constant atall times by the inner layer of insulation so that changes intemperature do not introduce capacity changes into the conductorassembly due to spacing changes between the inner and outer conductors.

It will also be seen that the inner conductor and the Faraday shieldform open electrical paths so that the inner conductor and the Faradayshield are connected to ground at the midpoints of the electric paths.

The foregoing description of the invention is explanatory only, andchanges in the details of the construction illustrated may be made bythose skilled in the art, within the scope of the appended claims,without departing from the spirit of the invention.

What is claimed and desired to be secured by Letters Patent is:

1. A circular inductor assembly for a metal detector including: acoaxial cable having an inner control conductor, an outer conductorcoaxial with and about said inner conductor, insulation means betweensaid inner and outer conductors and about said outer conductor, saidcable being disposed in a circular coil having a plurality of turns; aconductive shield disposed about the bottom portion of said coil; casingmeans about and enclosing said coil and said shield; and means forconnecting one end of said inner conductor, said outer conductor andsaid shield to an electric ground and for connecting the other end ofsaid inner conductor to an electric circuit.

2. The inductor assembly of claim 1, wherein said outer conductor andsaid shield constitute open electric paths, said connecting means beingconnected to said outer conductor and said shield at the midpoints ofsaid electric paths.

3. The inductor assembly of claim I, wherein said outer conductorcomprises a pair of sections of equal lengths, adjacent ends of saidsections being spaced from one another, said means for connecting saidouter conductor to an electric ground being connected to the ends ofsaid sections remote from said adjacent ends.

4. The assembly of claim 3, wherein said shield constitutes an openelectric path, said connecting means being connected to said shield atthe midpoint of its electric path.

5. The inductor assembly of claim 1, and a plurality of electric lightmeans mounted adjacent said inductor assembly, and means for connectingsaid light means to an input circuit for causing said light means toflash in accordance with the frequency of the voltage of said inputcircuit.

6. A detector including: a first oscillator means for providing anoutput having a stable reference frequency; a second oscillator having afrequency determining circuit for varying the frequency of the output ofsaid second oscillator; comparator means operatively associated withsaid oscillators for providing an output signal whose frequency variesin accordance with the difference between the frequencies of the outputsof said oscillator; and a circular inductor assembly comprising acoaxial cable having an inner central conductor, an outer conductorcoaxial with and about said inner conductor, and insulation meansbetween said inner and outer conductors and about said outer conductor,said cable being disposed in a circular coil having a plurality ofturns; a conductive shield disposed about the bottom portion of saidcoil; casing means about and enclosing said coil and said shield; meansfor connecting one end of said inner conductor, said outer conductor andsaid shield to an electric ground of said second oscillator, and meansfor connecting the other end of said inner conductor to said frequencydetermining circuit of said second oscillator, the frequency of theoutput of said second oscillator varying in accordance with theeffective inductance of said inductor assembly.

7. The detector of claim 6, wherein said outer conductor and said shieldconstitute open electric paths, said connecting means being connected tosaid outer conductor and said shield at the midpoints of said electricpaths.

8. The detector of claim 7, and indicator means responsive to the outputof said comparator means for providing a signal which varies inaccordance with the frequency of the output of said means.

9. The detector of claim 8 wherein said indicator means compriseelectric light means which flash in accordance with the frequency of theoutput of said comparator means.

10. A metal detector including: first and second circular inductorassemblies of different diameters and lying in a common plane, eachinductor assembly comprising a coaxial cable having an inner conductor,an outer conductor coaxial with and about said inner conductor,insulation means between said inner and said outer conductors and aboutsaid outer conductors, said cable being disposed in a circular coilhaving a plurality of turns, and a conductive shield disposed about thebottom portion of said coil; casing means about and enclosing said coilsand said shields of said inductor assemblies; a first oscillator havingan output of a stable reference frequency; a second oscillator having afrequency determining circuit; means for selectively connecting one endof said inner conductor of each of said inductor assemblies to saidfrequency determining circuit, the frequency of the output of saidsecond oscillator varying in accordance with the effective impedance ofthe inductor assembly whose inner conductor is connected to saidfrequency determining circuit; and comparing means operativelyassociated with said oscillators for providing an electric output signalwhich varies in accordance with the difference in the frequencies of theoutputs of said oscillators.

11. The metal detector of claim 10 wherein one of said inductorassemblies is disposed within the other of said inductor assemblies.

1. A circular inductor assembly for a metal detector including: acoaxial cable having an inner control conductor, an outer conductorcoaxial with and about said inner conductor, insulation means betweensaid inner and outer conductors and about said outer conductor, saidcable being disposed in a circular coil having a plurality of turns; aconductive shield disposed about the bottom portion of said coil; casingmeans about and enclosing said coil and said shield; and means forconnecting one end of said inner conductor, said outer conductor andsaid shield to an electric ground and for connecting the other end ofsaid inner conductor to an electric circuit.
 2. The inductor assembly ofclaim 1, wherein said outer conductor and said shield constitute openelectric paths, said connecting means being connected to said outerconductor and said shield at the midpoints of said electric paths. 3.The inductor assembly of claim 1, wherein said outer conductor comprisesa pair of sections of equal lengths, adjacent ends of said sectionsbeing spaced from one another, said means for connecting said outerconductor to an electric ground being connected to the ends of saidsections remote from said adjacent ends.
 4. The assembly of claim 3,wherein said shield constitutes an open electric path, said connectingmeans being connected to said shield at the midpoint of its elecTricpath.
 5. The inductor assembly of claim 1, and a plurality of electriclight means mounted adjacent said inductor assembly, and means forconnecting said light means to an input circuit for causing said lightmeans to flash in accordance with the frequency of the voltage of saidinput circuit.
 6. A detector including: a first oscillator means forproviding an output having a stable reference frequency; a secondoscillator having a frequency determining circuit for varying thefrequency of the output of said second oscillator; comparator meansoperatively associated with said oscillators for providing an outputsignal whose frequency varies in accordance with the difference betweenthe frequencies of the outputs of said oscillator; and a circularinductor assembly comprising a coaxial cable having an inner centralconductor, an outer conductor coaxial with and about said innerconductor, and insulation means between said inner and outer conductorsand about said outer conductor, said cable being disposed in a circularcoil having a plurality of turns; a conductive shield disposed about thebottom portion of said coil; casing means about and enclosing said coiland said shield; means for connecting one end of said inner conductor,said outer conductor and said shield to an electric ground of saidsecond oscillator, and means for connecting the other end of said innerconductor to said frequency determining circuit of said secondoscillator, the frequency of the output of said second oscillatorvarying in accordance with the effective inductance of said inductorassembly.
 7. The detector of claim 6, wherein said outer conductor andsaid shield constitute open electric paths, said connecting means beingconnected to said outer conductor and said shield at the midpoints ofsaid electric paths.
 8. The detector of claim 7, and indicator meansresponsive to the output of said comparator means for providing a signalwhich varies in accordance with the frequency of the output of saidmeans.
 9. The detector of claim 8 wherein said indicator means compriseelectric light means which flash in accordance with the frequency of theoutput of said comparator means.
 10. A metal detector including: firstand second circular inductor assemblies of different diameters and lyingin a common plane, each inductor assembly comprising a coaxial cablehaving an inner conductor, an outer conductor coaxial with and aboutsaid inner conductor, insulation means between said inner and said outerconductors and about said outer conductors, said cable being disposed ina circular coil having a plurality of turns, and a conductive shielddisposed about the bottom portion of said coil; casing means about andenclosing said coils and said shields of said inductor assemblies; afirst oscillator having an output of a stable reference frequency; asecond oscillator having a frequency determining circuit; means forselectively connecting one end of said inner conductor of each of saidinductor assemblies to said frequency determining circuit, the frequencyof the output of said second oscillator varying in accordance with theeffective impedance of the inductor assembly whose inner conductor isconnected to said frequency determining circuit; and comparing meansoperatively associated with said oscillators for providing an electricoutput signal which varies in accordance with the difference in thefrequencies of the outputs of said oscillators.
 11. The metal detectorof claim 10 wherein one of said inductor assemblies is disposed withinthe other of said inductor assemblies.
 12. The metal detector of claim11 and indicating means operatively associated with said comparatormeans for providing a signal perceivable by an operator of the detectorwhich varies in accordance with the frequency of the output of saidcomparator means.